1. Field of the Invention
This invention is directed to a frequency selective transient voltage protector.
2. Background of the Invention
Telecommunications systems operate over a wide range of signal voltage and frequency. In the United States, for typical communications (telephone) lines, a DC voltage of up to 60 volts is provided to the communications line to power terminal equipment and to act as a carrier for low voltage voice/data signals, and for ring signals. Analog telecommunications voice signals are typically in a frequency band ranging from 300 Hz to 5 KHz and are limited to approximately 5 volts peak. Digital voice signals and DSL (digital subscriber line) signals can have frequency content up to approximately 10 MHz and are also limited to approximately 5 volts peak. During a ringing interval, an AC signal with a frequency between approximately 15 Hz to 70 Hz and with a voltage of up to 150 volts RMS is provided to the communications line. The maximum voltage that can appear on a telephone line from the normal operation of the telecommunications system is 270 volts peak (sum of the peak value of the ring signal and the maximum DC voltage on the line and the peak value of a DSL signal if present).
For long telephone lines in the United States, loop extenders are sometimes employed which increase the DC voltage on the line up to 105 volts. However, lower voltage ring signals are typically used for long telephone lines, so that the peak operating voltage that can appear on the telephone line is still limited to 270 volts peak.
Conventional surge protectors are designed so that they do not operate unless the voltage on the telephone line exceeds the peak operating voltage of the communications system during the ring interval (typically 270 volts in the United States). These conventional surge protectors allow surge voltages up to at least 270 volts to pass through to sensitive low voltage terminal equipment attached to the communications line, when only low voltage signals are expected to be present on the communications line.
Current surge protectors, such as those disclosed in U.S. Pat. Nos. 4,941,063 and 4,758,920 (the ""063 and ""920 patent, respectively), the entire contents of each of those patents being hereby incorporated by reference, employ xe2x80x9cswitched filterxe2x80x9d technology to overcome many of the shortcomings of conventional surge protectors. Protectors of the type disclosed in the ""063 and ""920 patents employ a second stage that switches a filter onto the communications line if the voltage on the line changes by a fixed amount (typically 30 volts). Voice or data signals are of too low a voltage to activate the circuit. High voltage ring signals cause the filter to be switched onto the communications line but the filter time constant is chosen to have little effect on low frequency ring signals. Transients, which are comprised of both high voltage and high frequencies, are attenuated by the filter circuit.
Another over-voltage problem on a communications line is caused by lightning. Voltage surges on communications lines that are typically caused by nearby lightning strikes contain energy in the frequency band from DC to greater than 10 MHz, though most of the energy is contained in the frequency band between 25 KHz and 1 MHz.
Referring next to FIG. 1, a prior art voltage protection circuit is depicted and generally designated by reference numeral 100. In operation, when a signal is present on the communications line 10, a voltage is present across the communications line 10 when measured between the Tip and Ring. When the change in voltage across the communications line 10 exceeds the breakdown voltage of CR1 (typically 30 volts), CR1 enters its conductive state (essentially a short circuit) and connects C1 across the communications line 10. That condition preferably occurs when a high voltage transient signal is present on the communications line 10. C1 forms a filter with R1 and R2 and filters the voltage present on the line until the current through CR1 reduces to a value below the holding current rating of CR1. When the current through CR1 reduces to such a value, CR1 returns to its high impedance state (essentially an open circuit) and disconnects capacitor C1 from across the communications line 10. The values of C1, R1 and R2 are chosen to present a high impedance at the frequencies employed for ring signals. Thus, if the voltage change on the line 10 was caused by the presence of a ring signal, the filter has little effect on the ring signal because of its high impedance at the low frequencies used for ring signals.
Transient voltages that may be present on a communications line have significant amounts of energy at frequencies that are considerable higher than the frequencies used for ring signals. In FIG. 1, the impedance of C1 is inversely proportional to frequency. If the voltage change on the line 10 was caused be the presence of a transient voltage (which has a high frequency component), the filter comprised of C1, R1 and R2, has a large effect on the transient voltage because of its low impedance at the high frequencies that are present in transients on communications lines.
However, use of the circuit depicted in FIG. 1 on communications lines that have POTS and DSL service operating simultaneously on the same line, may result in attenuation of a DSL signal. Under those circumstances the voltage change from the ring service of POTS causes the filter to connect across the line. This has little effect on the ring signal, but presents a low impedance to the high frequency DSL signal causing significant attenuation of the DSL signal for the duration of the ringing period.
It is thus desirable to provide a voltage protection circuit that overcomes the above-described shortcomings of the prior art, and that may be used on a telecommunication line over which both POTS and DSL may be present simultaneously.
In an embodiment of the present invention, the FSTVP circuit comprises a frequency discriminator connected across the communications line, a voltage discriminator connected to the frequency discriminator, and a overvoltage protection device connected to the voltage discriminator. Preferably, the frequency discriminator comprises a capacitor and resistor connected together in series across the communications line or, alternatively, a resistor and inductor connected together in series across the communications line. The voltage discriminator preferably comprises a solid state thyristor-type device, such as a PNPN structure, self-gated triac, or other type of symmetrical transient voltage suppressor device or various other devices that may be combined to achieve the desired voltage discrimination in accordance with the present invention and as described in detail herein. The overvoltage protection device may be any device having at least high impedance and low impedance operating states, and that may be caused to switch between the high and low impedance states (either from high to low, or visa versa) under certain predetermined condition(s). For example, the overvoltage protection device may be a uni- or bi-polar device, a silicon controlled rectifier (SCR), a triac, a p-gate thyristor, a transistor, or other known or hereafter developed device that provides the same or similar functionality to the previously listed devices and as otherwise described herein. The FSTVP circuit of the present invention may also comprise a filtered output and DC overvoltage protection devices to provide shunt paths for low frequency, high voltage transients.
The FSTVP circuit of the present invention thus permits a low frequency signal, such as a ring signal, to pass unattenuated. At the same time, the FSTVP circuit of the present invention permits a high frequency, low voltage signal, such as a DSL signal, to also pass unattenuated. However, the present invention may attenuate (partially or completely) a high frequency, high voltage signal, and a low frequency, high voltage signal such as a transient voltage, so as to prevent damage to service personnel and to devices connected to the communications line protected by the FSTVP circuit.
The FSTVP circuit of the present invention may also be used in connection with other components, circuits and devices. For example, DC overvoltage protection components may be connected to the inventive FSVTP circuit, the output of the FSTVP circuit may be filtered (using a RC or LC circuit), and components may be added to the FSTVP circuit to facilitate the use of uni-polar overvoltage protection devices. The various combinations and embodiments of the present invention will be discussed in more detail below.
The invention accordingly comprises the features of construction, combination of elements, and arrangement of parts which will be exemplified in the disclosure herein, and the scope of the invention will be indicated in the claims.